Subscriber signaling system



Oct. 20, 1953 R. H. HERRICK ETAL SUBSCRIBER SIGNALING SYSTEM 1'0 sheets-'sheet l Filed Feb. 12. 1949 RoswELL H. HERRICK ERwlN M. RoscHKE n INVENTOR.

l an@ TH DR AGENT Oct. 20, 1953 R. H. HERRICK ET AL SUBSCRIBER SIGNALING SYSTEM Filed Feb. l2, 1949 10 Sheets-Shelet 2 Roswl-:LL H. HERRlcK ERwlN M. RoscHKE INVENTOR.

r N E G A /R va. H @T Y B Oct; 20, 1953 R. H. HERRICK ETAL 2,656,407

SUBSCRIBER SIGNALING SYSTEM Filed Feb. 12, 1949 10 Sheets-Sheet 3 '.LREV. OF CODE TAPE TIME-* RoswELL H. HERRlcK ERwlN M. RoscHKE INVENTOR.

'THEIR AGENT Oct. 20, 1953 R. H. HERRICK ETAL SUBSCRIBER SIGNALIN'G SYSTEM l0 Sheets-She'et 4 Filed Feb. l2, 1949 RoswEu. H. HERRICK ERwlN M RoscHKE IN VEN TOR.

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THR AGENT Oct. 20, 1953 R. H. HERRICK ET AL SUBSCRIBER SIGNALING SYSTEM 10 Sheets-Sheet 5 Filed Feb. 12, 1949 Roswr-:LL H. HERRICK ERWIN M. RoscHKE INVENTOR.

TH EKR AGENT mad Or-F200 R. H. HERRICK ET AL SUBSCRIBE-IR SIGNALING SYSTEM oct. 2o, 1953 10 Sheets-Sheet 6 *IIIHC RoswELL H. HERRICK ERwIN MRoscHKE mm E .Om F152 Filed Feb. 12. 1949 .om El A SmSo mw15n IN V EN TOR.

T. N E G A R E H T l0 Sheets-Sheet 7 Filed Feb. 12. 1949 Oct. 20, 1953 R. H. HERRICK ET A1. 2,655,407

SUBSCRIBER SIGNALING SYSTEM Filed Feb. 12, 1949 10 sheets-,sheet e Oct. 20, 1953 R. H. HERRlcK ET AL 2,656,407

SUBSCRIBER SIGNALING SYSTEM Filed Feb. l2, 1949 10 Sheets-Sheet 9 Il! OQO t-m o giri l0^ (nu-l no c v L o O N FIG. 7A

SWITCH DEVICE GENER ROswELL H. HERRICK ERwlN MROSCHKE THE 1FL AGENT Oct. 20, 1953 R. H. HERRICK `ET A1. 2,656,407

SUBSCRIBER SIGNALING SYSTEM Filed Feb. 12, 1949 1o sheets-sheet 1o o f C) RECTIFIED PULSES EQUALIZER FIG. 8

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SYNCH.

PULSES HOSWELL H. HERRICK T/\O hl ERwlN MROSCHKE IN VEN TOR.

THEIR AGENT I9 RECEIVE L TAPE Patented Oct. 20, 1953 SUBSCRIBER SIGNALING SYSTEM Roswell H. Herrick, Lorain, Ohio, and Erwin M. Roschke, Broadview, Ill., assignors to Zenith Radio Corporation, a corporation of Illinois Application February 12, 1949, Serial No. 75,988

16 Claims.

This invention relates to coded electrical signalling systems of the subscription type, and more particularly to such systems in which coded electrical signals are transmitted on one channel, and key signals for decoding the electrical signals are transmitted solely to authorized subscriber receivers on a second channel.

The accompanying drawings and associated text describe the technical details and method of operation of a subscription type of television system embodying the instant invention. However, it may be 'helpful to have as background information a simplified explanation of the underlying principles of that system to facilitate a quick understanding of the illustrated arrangements.

In accordance with one form of the invention, a television transmitter is provided with a coding arrangement which, in response to an applied signal, is capable of changing the operation of the transmitter from one mode to another, the change in modes constituting a coding function to introduce an aspect of privacy by virtue of which unauthorized receivers are not able to utilize the transmitted program signal. nal which may have a random amplitude is established in a suitable vehicle, such as a tape, a record disc, a memory tube, or the like, so that a scanning element or other pick-on device may recurrently derive the recorded or stored signal. This derived signal is applied to the coding arrangement to effect a change from one mode of operation to another in the transmitter in a prescribed coding schedule represented by the amplitude variations of the control signal. Preferably, the cycle of the coding schedule is very short wit-h respect to any program period. Hence, the transmitted signal is coded in accordance with a repeating coding schedule many times in a given program.

Obviously, it is necessary that subscriber receivers be supplied with a key to enable their utilization of the otherwise unintelligible transmitted signal. To that end, it is contemplated in the present invention that the subscriber place L A control sig- Kil utilized in the transmitter. Moreover, the receiver is provided with apparatus which is complementary to the coding arrangement of the transmitter and which is subject to the influence of a key signal to decode the received but scrambled transmission. A key signal, for application to the decoding arrangement, is obtained by scanning the record or storage unit in the receiver. Thus, considering the receiver alone, its decoding arrangement receives a key signal that represents the coding schedule of the scrambled transmission and responds to that key signal to effect compensating changes in the mode of operation to accomplish decoding. The decoding thus obtained permits the receiver to utilize the received program signal and reproduce intelligible images.

For cooperative action on the part of the transmitter and receiver, it is necessary that their operations be precisely correlated. It has already been explained that the coding at the transmitter and the decoding at the receiver are fully complementary but synchronizing and phasing must also be realized. The synchronizing is essentially simple and may be satised by synchronizing the scanning at the transmitter and receiver to operate at identical and synchronous speeds. Phasing is attained by comparing the time relation of corresponding portions of the coding cycles at the transmitter and receiver. For that purpose, a special phasing signal may be sent out from the transmitter at the start of a coding cycle and preferably during a field retrace interval so that it has no adverse effect on the picture information. Likewise, a phasing signal may be provided in the record or storage device at the receiver to represent the start of its coding cycle. These phasing signals may be applied to what is essentially a phase detector which provides a control eiect in accordance with the time relation of the compared phase signals in any coding cycle. That control eiect may adjust the scanning at the receiver to maintain proper phase as well as synchronizing relations. Although the coding information is supplied to the receiver usually prior to the program interval, the phase correction alluded to takes place during the assigned program interval so that the authorized receiver may enjoy the transmission to which it has subscribed, whereas unauthorized receivers experience wholly unintelligible reception of such programs.

Subscription type signalling systems have been proposed wherein television signals are radiated in coded form, and key signals for decoding the coded signals are transmitted to authorized subscriber receivers over suitable line circuits, such as the telephone lines. The key signals are supplied to individual subscribers upon request and a suitable charge is made for the use thereof. Such systems, as heretofore proposed, require the continuous use of the telephone lines as a transmission link for the key signals, and have been objected to on the ground that they unduly interfere with the normal service of the telephone system.

The present invention provides a subscription type of signalling system in which coded signals are transmitted to subscriber receivers, and key signal information required to enable the receivers to decode the coded signals for a long period of time is supplied thereto over the telephone lines, but in an extremely short time interval and by means of an essentially normal telephone call. In this manner, continuous use of the telephone lines for supplying key signals throughout entire program intervals is avoided, and no objection may be raised to the eiect that the system interferes with the normal service of the telephone line.

It is, therefore, an object of this invention to provide a. coded signalling system of the subscription type in which key signals for decoding the coded signals are produced and distributed toV subscriber receivers in an improved and more convenient manner than in previous systems of this type.

Another object of this invention is to provide such a system in which suicient key signals for decoding the coded program signals for a relatively long period of time are produced and distributed to individual subscriber receivers in a relatively short time interval, preferably, before the transmission of the coded program signals, whereby more efiicient use is made of the key signal channel, thatv is, this channel is not used continuously by an individual subscriber but for short time intervals only, leaving the channel free for other purposes and for use by other subscribers during periods when it is not in use by the rst mentioned subscriber.

Yet another object of this invention is to provide such a system in which use may be made lof telephone networks to distribute the key signal to subscriber receivers, whereby each subscriber uses the telephone network for an extremely short time interval to obtain suflicient key signals to decode the received coded signals for a program that may extend over a relatively long period of time.

A further object of this invention is to provide a receiver for use in such a coded signalling system, which receiver is capable upon the receipt of a key-signal pattern to utilize this pattern recurrently to decode the received coded signals.

The features of this invention which are believed to be new are set forth with particularity in the appended claims. The invention itself, however, together with further objects and advantages thereof may best be understood by reference to the following description when taken in conjunction with the accompanying drawings, in which:

`Figure 1 shows a transmitter which incorporates the present invention,

Figure 1A shows a modification of a portion of the transmitter of Figure 1,

Figure 2 shows diagrams useful in the understanding of the operation of the transmitter of Figure 1,

Figures 3 and 4 showin detail the circuits of certain components of the transmitter of Figure 1,

Figure 5 shows diagrams useful in the understanding of the operation of the circuit of Figure 4,

Figure 6 shows a convenient system for use as a central distributing station,

Figure 7 shows a receiver for operation in conjunction With the systems of Figures 1 and 6,

Figure 7A shows a modification of a portion of the receiver of Figure 7, and

Figure 8 shows in detail the circuit of one of the components of the receiver of Figure 7.

Copending application Serial No. 742,374, filed April 18, 1947, entitled Radio Wire Signalling System, by Alexander Ellett et al., which has now issued as Patent 2,510,846, May 30, 1950; and copending application Serial No. 773,848, filed September 13, 1947, entitled Image Transmission System, By Erwin M. Roschke, now issued as Patent 2,547,598, April 3, 1951, both assigned to the present assignee, disclose typical coded electrical signalling systems of the subscription type.

In these systems, coded program signals are radiated and key signals for decoding the coded signals are distributed to subscriber receivers over suitable wire line conductors. In such systems the key signals are transmitted continuously over these conductors to the subscriber receivers throughout the entire program period. In accordance with the present invention, sufficient key signal information for decoding the coded electrical signals for long periods of time are transmitted to the subscriber receivers in a brief time interval, and not continuously. Hence, when a telephone network is used for the distributionof the key signals, a subscriber may obtain surlicient inormation by means of a short telephone call, to enable his receiver to decode the coded signals for a long program interval, for example, for several hours.

The present invention will be described as applied to a coded subscription signalling system, such as the system disclosed in application Serial No. 773,848. However, it is to be understood that the invention may similarly be applied to other types of such systems, such as those disclosed in application Serial No. 742,374. The systems of Figures 1 and 7 are similar in many respects to the system shown in application Serial No. 773,848 and many of the components of the systems of Figures 1 and 7 have been described in detail in that application. Since these components in themselves form no part of the present invention, a detailed description thereof is believed to be un.- necessary.

Referring now to Figure 1, the transmitter there represented schematically includes a camera l having mounted therein a television picture converting device 2 of the iconoscope, image orthicon, or any other suitable type. The camera I includes a lens system 3' for focusing .images of scanned subjects on the device 2.

of leads Il) and to a vertical sweep generator Il by Way vof leads i2. The generator 9 is also lcoupled to va horizontal sweep generator I3 through a delay line and switch I4. The vertical and horizontal sweep generators are respectively coup'led to the vertical sweep coils I5 and to the horizontal Isweep coils I6 of device 2. The generator 9 is further coupled to a random frequency divider Il which may be of the type disclosed and claimed in copending application `Serial No. 32,457, Erwin M. Roschke, entitled "Random Frequency Divider, led June 11, 1948, now issued as Patent 2,588,413, March 11, 1952, and assigned to the present assignee. The divider I1 is connected through a switching device I8 to a key signal oscillator I9. Generator 9 is also coupled to a key signal filter and switch operator 20 b-y way of leads 2I, the switch operator 20 being connected to the delay line and switch I4 by means of three leads 22.

In the operation of the system thus far described, video-frequency signals representing a scanned object are produced by the device 2. These signals are amplied in the video yamplier 4 and the amplied -signals are mixed in the mixer amplifier 5 with vertical and horizontal synchronizing pulses and blanking pedestals from the generator 9. The resulting composite television signal from mixer amplifier 5 is passed through the background reinsertion -device 6, and this signal, properly adjusted as to background level, is modulated on a suitable carrier wave in stage 1, the modulated carrier wave being radiated from the antenna 8. The vertical and horizontal scannings of the device 2 are controlled respectively by the sweep generators II and I3. synchronized directly by generator 9 through leads I2, and the horizontal sweep generator I3 is synchronized by generator 9 through the delay line and switch I4. The delay yline and switch I4 has two positions, and when in one of these positions this stage passes the horizontal synchronizing pulses directly to the generator I3, and when in the other of these positions these pulses are delayed a certain time interval. The delay line and switch I4 is switched from one position to the other by the switch operator 20 through leads 22, and this switching at spaced time intervals causes the horizontal synchronizing pulses passing therethrough to be delayed during such intervals. This delay causes the generator I3 to delay the timing of the horizontal scanning of device 2 and, hence, during these spaced time intervals the video-signal components of. the radiated television signal are delayed With respect to the synchronizing-signal components thereof, thus effectively coding the vradiated signal.

Vertical synchronizing pulses from the generator 9 are randomly frequency divided in the frequency divider Il, and when the switching device I8 is closed these pulses cause the key signal oscillator I9 to generate bursts of key signal, each burst corresponding to a pulse from divider I1. In a manner to be descri'bed, these bursts are recorded for a predetermined interval to form a key signal pattern in a recording device, or other suitable signal storage device, and this pattern from the recording device is impressed recurrently as a coding signal on the switch operator 29 over the leads 23 which connect the recording device to the switch operator. The key signal filter and switch operator 2u, in response to the coincidence of a burst of key signal on the leads '23 :and a vertical synchronizing pulse on the leads 2|, ac-

The vertical sweep generator II is tuates the delay line and switch I4, such actuation thereby commencing durng vertical retrace intervals of the device 2. In this manner, changes in the television signal from a normal mode to a mode in which the video signal is delayed with respect to the synchronizing signal due to actuation of the delay line and switch I4 take place during these vertical retrace intervals and -distortion in the image reproduced in subscriber receivers is thereby avoided.

A detailed description of the above described type of coding systemis given in application Serial No. 773,848, together with a description of the circuits and operation of the various components, and further description herein is believed to be unnecessary. As previously stated, the present system differs from the system of the above application in that signals from oscillator I9 are not impressed continuously on the switch operator 29 over the leads 23, but these key signals are recorded for a certain time interval to obtain a key signal pattern, and this pattern is impressed recurrently as a coding signal on the switch operator 29 over the leads 23 during the program interval when the coded television signal is radiated from antenna 8.

The term key signal pattern is used to refer to a number of key signal bursts from the key signal oscillator I9 which `are recorded and used recurrently as a coding signal to effect coding of the television signal.

The following is a detailed description of the means whereby the key signals from oscillator 9 are recorded to obtain the key signal patern.

Vertical synchronizing signals from the generator 9 are used to synchronize the generator 24 to the frequency of these synchronizing signals, or some harmonic thereof, and the generator 24 drives a motor 25. This motor, in turn, drives a recorder which is shown to comprise end-1 less magnetic tapes 26, 2l and 28. These tapes, and others to be described herein are, preferably, rotated by the driving disks by means of sprockets such as the nlm in motion picture projectors to insure that there is no slippage. Furthermore, the illustrated recorders may be replaced by any known type of signal storage device, such as memory tubes, disk or wire recorders, and the like. The pulses from the frequency divider Il are passed through the switching device I8 to the key signal oscillator I9. Switching device I3 is arranged, in a manner to be described, so that when it is closed, pulses from the frequency divider I 'I energize the oscillator I 9, and generated oscillations are recorded on the tape 2l through a recorder head 29, the tape 2l bein-g designated as the code tape. Switching device I8 is so controlled that it remains closed for not more than one revolution of the code tape 2l and a pattern of key signals from oscillator I9 is thereby recorded on this tape. The key signal pattern recorded on tape 2l is used recurrently to actuate switch opera-tor 20, this pattern being impressed on the switch operator over leads 23, which leads extend to the operator from a pick-up head 39 associated with the code tape 2l. The pattern o-f key signals from the pick-up head 39 is further impressed through an amplifier 3l on conductors 32, forming a line circuit leading to a central distributing station, to be described. Conductors 32 may be any form of a link between the transmitter and the distributing station, and, when desired, the pattern of key signals may be radiated to this station Afrom the transmitter, thus dispensing with these conductors.

It is necessary that'l switching device I8 close and open in an interval between the pulses from frequency divider I1 and also that this switching device remain closed for no more than one revolution of the code tape 21. To accomplish this, tape 28 which is designated the switch tape is rotated by motor at a speed much slower than tapes 26 and 21, by means of a reduction gear 33, tapes 26 and 21 rotating preferably at the same speed. For example, when the tapes 26 and 21 are rotated at the preferred speed of `60 revolutions per minute, tape 28 may be rotated at one revolution per minute. A switching signal having a frequency, for example, of 3,000 cycles per second is recorded -by any known means on the switch tape 28, and this tape is adjusted so that this switch signal is phased with respect to the vertical synchronizing signals of the system to close the switching device I8 in an interval between succeeding pulses from the frequency divider I1. 'I'he duration of the switching signal recorded on the switch tape 28 is made such that when this signal is impressed on the switching device I8, this device is closed for one revolution only of the code tape 21. Therefore, when it is desired to record a key signal pattern on the code tape 21, o-r to change an existing pattern on this tape, a switch 34 in the leads connecting an erasing signal source 36 to the erasing head 31 associated with the code tape 21 is closed for several seconds in an interval before the leading edge of the switch signal on the switch tape 28 reaches the pick-up head 35 associated with the switch tape. Closing the switch 34 applies an erasing signal from the source 36 to the erasing head 31, and an existing signa-l on the code tape 21 is thereby erased. Switch 34 is then opened and a switch 38 in the leads connecting the pick-up head 35 to the switch device I8 is closed. Now, when the switching signal on the switch tape 28 reaches the pick-up head 35, the switch device I8 closes and passes pulses from the divider I1 to the key signal osy cillator I9. The switch device I 8 is maintained closed by the switch signal from the switch tape 28 for one revolution of the code tape 21, and during this revolution of the code tape a new key signal pattern is recorded thereon. When the v code tape 21 has completed the revolution, the trailing ed-ge of the switch signal recorded on tape 28 passes the pick-up head 35, and the switch devicev I8 opens. This new key signal pattern may thereafter be picked up by the pick-up head 30 associated with the code tape 21, and applied recurrently to the switch operator 20 over the leads 23 to actuate this operator, as previously described.

Therefore, in the sys-tem thus far described, a coded composite television signal is radiated from antenna 8 in which, during spaced time intervals, the video-frequency components are delayed a certain amount with respect to the synchronizing-signal components of this composite signal, and in which the change from a normal mode of the television signal to a mode in which the video-frequency components are so delayed takes place during vertical retrace intervals. This coding of the television ysignal is effected by the switch operator 20 in response to the coincidence of vertical synchronizing pulses on leads 2l, and respective key signal bursts of the key signal pattern on leads 23. The described system also provides for the transmission of the key signal pattern to subscriber receivers over the line circuit 32 and through a central distributing station.v

The tape 26 is designated the control tape, and as previously mentioned, this tape is preferably driven at the same rate of speed as the code tape 21. The purpose of the control tape 26 is to provide a control signal so that a decoding tape at a subscriber receiver may be fi synchronized with the code tape 21, in a manner to be described. The control signal is recorded by any known means on the control tape 26, and the control tape is adjusted so that this control signal is phased to indicate preferably the point on the code tape 21 of the commencement and termination of the key signal pattern recorded on the last mentioned tape. The control signal recorded on the control tape 26 is given a frequency substantially different than the frequency of the key signal bursts recorded on the code` tape 21 and the control signal is transmitted to subscriber receivers with the key signal over the line circuit 32 and through the central distributing station.

The control signal recorded on the control tape 26 is also used to cause a second control signal to be transmitted to the subscriber receivers over the same channel asthe coded television signals. In a manner to be described. the subscriber receivers utilize the time coincidence of the two control signals 'to eilect synchronism between decoding tapes at the receivers and the code tape `21. As stated, the first control signal is transmitted to the subscriber receivers over the line circuit 32, this signal being pickedup from the control tape 26 by a pick-up head 40 associated with this tape and amplirled in an amplifier 35 that is sharply tuned to the frequency of'this signal and has its output terminals coupled to the line circuit 32 to apply this control signal thereto. Furthermore, the control signal from the pick-up head 40 is impressed on a control signal phasing circuit 4I over the leads 42. The circuit 4I acts in a manner to be described, to pass at proper intervals vertical equalizer pulses from generator 9 to the mixer amplier 5, these pulses having been frequency multiplied in a frequency multiplier 43. The pulses passed by the circuit 4I form the second control signal and this signal is transmitted to subscriber receivers over the television signal channel.

In the embodiment of Figure 1, the various opera-tions of erasing a key signal pattern from the code tape 21, and recording a new pattern thereon are preferably completed before actual transmission of the television program by the system. It is contemplated in this embodiment that a key signal pattern on the code tape 21 be used recurrently to code the television signal for an interval corresponding to one or more television programs, for example, for one or two hours. After this program interval, the pattern is changed and a new pattern recorded on the code tape 21 for effecting coding during the next succeeding program interval. Hence, it is necessary to disable the transmitter between such program intervals, so that an existing key signal pattern may be erased and a new pattern recorded on the code tape 21.

Figure 1A shows a modication of the system of Figure 1, and in the modified system a plurality of code tapes 21a, 21h, '21c each have a different key signal pattern recorded thereon. The diiferent key signal patterns may be reterval.

corded on the respective code tapes by means of the apparatus shown and described in the system of Figure 1, this recording being effected before actual transmission of the television signal, that is before any of the program intervals.

Amplifiers 3Ia, 3Ib, 3Ic have input terminals connected respectively to the pick-up heads a, 30h, 30e associated respectively with the code tapes 21a, 21b, 21C; and the output terminals of these amplifiers are connected to respective line circuits 32a, 32h, 32e extending to the central station (not shown). The key signal patterns on the code tapes are amplified in the respective ampliers and are transmitted over the line circuits to the central station. For a certain program interval, for example, for the first television program, a switch 44a, is closed, and

' the pattern from the code tape 21a is used to code the television signal during the interval of this program. At the end of this program, switch 44a may be opened and switch 442) closed, and the pattern from code tape 21h used to code the television signal for the next succeeding program interval. Similarly, the pattern on the code tape 21o may be selected and used to code the television signal for an ensuing program in- In this manner the coding of the television signal may be changed for each program or group of programs, without the need of disabling the transmitter to erase an existing signal pattern from the code tape and to record a new pattern thereon. It will be understood that the selector switches 44a, 44h, 44o may be actuated by any well known form of time control apparatus to accomplish a change from one code tape to another automatically and at preselected time intervals.

Figure 2 shows graphically the various signals recorded on the tapes driven by the motor 25 of Figure 1, and the relation of these signals to each other, to the vertical synchronizing pulses from generator 9, and tothe pulses from random frequency divider I1. Referring now to Figure 2, the vertical synchronizing pulses are shown in curve 2A, and these pulses occur at the indicated intervals. The vertical synchronizing pulses of curve 2A are impressed on random frequency divider I1 of Figure '1, and this divider acts, as described in previously mentioned copending application Serial No. 32,457, to select at random, certain of the vertical pulses, such selected pulses being shown in curve 2B. The signal recorded on control tape 26 of Figure 1 is illust-rated in curve 2C. This signal vis phased to be nrst picked-up by head and applied to switching device I6 at the same instant that the control signal of curve 2C is first picked-up by head assuming that switch 38 is closed. The switching signal is shown in curve 2D and continues for a complete revolution of the control tape 26 and the code tape 21, these latter two tapes rotating at identical speeds. Hence, switching device I8 is closed in the interval between the vertical synchronizing pulses 45 and 46, remains closed for a complete revolution of the code tape, and is opened by the termination of the switching signal of curve 2D, the trailing edge of the switching signal passing the head 35 in the interval between vertical synchronizing pulse 46 and the next succeeding vertical pulse. When switching device I8 is closed, the pulses of curve 2B are impressed on key signal oscillator I9 and a burst of key signal from this oscillator is recorded on code tape 21 in response to each pulse. Hence, pulses 49-53 of curve 2B cause bursts of key signal 54--58 of curve 2E respectively to be recorded on the code tape 21, the burst 58 occurring just before the switch signal of curve 2D causes switching device I6 to open, sufficient time being allowed so that the entire burst 58 is recorded on the code tape 21. The bursts 54-58 recorded on the code tape constitute the key signal pattern and as previously described, switch 38 is then opened, and this pattern is used recurrently to actuate the iilter and switch operator 20 of Figure 1 until such time that it is desired to change the coding pattern of the system.

The switching device I8 is shown in detail in Figure 3. The switch signal from pick-up head 35 of switch tape 28 lof Figure 1 is impressed across terminals 59, one of the terminals 59 being grounded and the other being coupled to a control electrode 60 of an electron discharge device 6I through a capacitor 62. The control electrode B0 lof device 6I is connected to ground through a grid leak resistor l63, and the cathode 64 oi.' this device is connected directly to ground. The anode 65 of device 6I is connected to a source 66 of unidirectional potential through a tuned circuit 61, the negative terminal of source 66 being grounded. The tuned circuit 61 consists of an inductor 68 shunted by a capacitor 69, this circuit being sharply tuned to be resonant at the frequency of the switching signal from tape 28 of Figure 1. The circuit of ldevice 6I amplifies the switching signal, and discriminates against signals having frequencies other than the frequency of this switching signal. The amplified switching signal from device 6I is rectified by a rectifying device 10, this rectifying device having a cathode connected to anode 65 of device 6 I, and an anode connected to a control electrode 1I of an electron discharge device 12. Cont-rol electrode 1I of discharge device 12 is connected to the common junction of series connected resistors 13 and 14, these resistors being connected across source 66 as shown. 'I'he anode of device 10 is coupled to ground through capacitor 15. The anode 16 of discharge device 12 is connected to the positive terminal of source 66, and the cathode 11 of this device is connected to the cathode 1B of a discharge device 19, the cathodes 11 and 18 being connected to ground through a. common resistor 80. The device 12 is so arranged that in the absence of the switching signal across terminals 59, this device is highly conductive due to the positive bias on its control electrode 1I from the potential divider action of resistors 13 and 14. The space current flowing in device 12 1 causes a current iiow through common cathode resistor 80, hence increasing the potential on cathodes 11 and 18. The device 19 is so arranged that under these circumstances this device is rendered non-conductive. When the switching signal is applied across terminals 59, this signal, ampliiied by device 6I and rectified by device 10, decreases the potential on control electrode through a grid leak resistor 84.

later circuit.

vthrough an adjustable resistor I -xfll of device 12, and accordingly decreases the space current owing in device 12. ythe current through common cathode resistor 80 also to decrease, and the potential of cathode 18 drops to a point where device 19 becomes con- This causes ductive. Pulses of positive polarity from frequency d1- .vider I1 of Figure 1 are impressed across terminals 8|, one of these terminals being grounded .'-and the other being coupled to control electrode 82 of device 19 through a capacitor 83, and control electrode 82 being connected to ground The anode 85 of device 19 is connected to the positive terminal of source 66 through load resistor 80, and this vvanode is connected to one of a pair of output terminals 81, the other of these terminals being grounded. Output terminals 81 are connected -ito the key signal oscillator |9 of Figure 1. I the absence of the switching signal across terminals 59, the device 19 is non-conductive, and lthe frequency :divided pulses impressed across terminals 8| do not appear at terminals 81. However, when the switching signal appears at terminals 59, the pulses at terminals 3| are amplied by device 19, and these amplified pulses are obtained across terminals 81 to actuate key signal oscillator I9 during spaced operating intervals. The control signal phasing circuit 4| of Figure i 1 is shown in detail in Figure 4, and diagrams explaining its .operation are shown in Figure 5. In the circuit of Figure 4, vertical blanking pulses from generator 9 of Figure l are impressed across terminals 88, these pulses having a form such as shown in curve 5A. The blanking pulses 1mpressed across terminals 88 are ,differentiated in a. differentiating .circuit Whichincludes a capacitor 89 and a resistor 90, the differential pulses having the form shown in curve 5B. These differentiated pulses are applied to the control electrode 9| of an electron discharge device, 92 through a Winding 93 of a transformer 9,4 and through a coupling capacitor 95, control electrode 9| oi device 92 being connected to ground through an adjustable resistor 9.6. The discharge device 92 is connected as a conventional blocking oscil- The cathode 91 of this device is connected to ground through a cathode resistor 98 shunted by a capacitor 9,9. The anode |00 of I device 92 is connected to the positive terminal of a source of unidirectional potential through a. further Winding |02 of transformer 9,4 and through a load resistor |03. Pulses of negative polarity appear at the anode |00 of device 92,

and the blocking oscillator is so adjusted that these pulses have the form and duration shown in curve C. The pulse output of the first .blocking oscillator is differentiated by a diierentiating circuit consisting of a capacitor |04 and a resistor |05, and the differentiated pulses, Shown in curve 5D, are used to trigger a second blocking oscillator. The differentiated pulsesare impressed on the control electrode |06 of an electron discharge device |01 through a Winding |08 of a transformer |09 and through a coupling capacitor |10,

control electrode |09 being connected to `ground The cathode I l2 of `device |01 is connected to ground through a cathode resistor |I3. The anode ||4 of device |01 is connected to the positive terminal of source |0| through further Winding |I5 of transformer |09. The positive peaks of the differentiated l pulses, shown in curve 5D, which peaks corre- .spond to. the trailing edges of the pulses from l2 the first block-.ine oscillator, trigger the second .blocking Oscillator .in the usual manner. the seeond blocking oscillator being adjusted to produce pulses of positive polarity, as shown in cur-Ve 5E, across its cathode resistor l, 3.

The control signal from pick-up head 40 of control tape 26 of Figure 1 is shown in curveG and is impressed across terminals IIB. One of the terminals ||6 grounded and the other :is coupled to the control electrode ||1 .of an electron discharge device |18 through a capacitor H9, control electrode ||1 `being connectedto ground through a grid leal; `resistor |120.l The cathode I2l of device |18 is connected to ground. and anode |22 of this device. .is connected t0 the positive terminal of a Source l 23 0f unidirectional potential through a tuned circuit |24, 4lconsisting ofy en indutor |25 shunted by a capacitor |26, and the negative terminal of Source |23 is grounded. The circuit |24 is tuned to be resonant at the frequency rof the control signal impressed .across terminals IIE, and the circuit of device I6 amplies this control signal and discriminates against signals having frequencies other than that of rthe control signal.

The amplified control signal from device I| i8 is rectified by means of a rectifying device |21. The anode of device |21 is connected tothe anode |22 of device H8, and the ,cathode of device |21 is coupled to ground through a `capacitor |28 and a shunt connected resistor |29. A positive unidirectional potential, as shown in curve 5H, appears across vresistor |29 in response to the amplified control signal, andv this potential is applied to the control electrode |30 of an electron discharge dev-ice |3I. Cathode. |32 of device |3| is connected to ground through a resistor |33 shunted by a capacitor |34. Cathode |32 is further connected to the positive terminal of source |23 through a resistor |35, and this cathode is positively biased by the potential divider action of resistor |35 and resistor |33. vScreen electrode |36 is connected to the positive terminal of source |23 through a resistor |31, and this electrede is coupled te ground. through. a byf-pass eapacitor |38.

Pulses from the second blocking oscillatonshown in curve 5E, are impressed on a second control electrode |39 of device 3| from cathode H2 of device |01 through a coupling capacitor |40, this control electrode being connected to ground through a grid leak resistor |4|. Device |31 is s o biased that this device becomes conductive only at such intervals when the rectified control sig-nal, as shown in curve 5H, is impressed on control electrode |30 at the `saine time that the pulses, show-n in curve 5E, are impressed on control electrode |39. At such intervals` of signal coincidence, the potential of anode |42 drops `due to the cur-rent iiow through load reSiSrtor |43, which resistor connects this anode to the positive terminal of source |23.

The anode |42 of device |3| is coupled to the control electrode |44 of an electron discharge device |45 through a coupling Capacitor |46, the control electrode |44 being connected -to vground through a grid leak resistor |41. The anode |148 of device |45 is connected to the positive terminal of source |23, vand the control electrode |44 of this device is also connected -to this positive terminal through a resistor |49. The cathode |50. of device |45 is connected to the cathode I 5| of an electron discharge device |52, and these ,cathodes are connected to ground through a common cathode resistor |53. When Ldischarge device INilis in its non-conductive state and the anode |42 of this device is at a relatively high potential, the control electrode |44 of device |45 is biased positively due to the potential divider action of resistors |49 and |41, and under these circumstances a large amount of space current ilows in device |45. This now of space current causes the discharge device |52 to be non-conductive due to the potential drop across the common cathode resistor |53. However, when the anode |42 of device |3| drops to a relatively low potential value due to the conductivity of device |3|, the positive bias on control electrode |44 of device |45 is reduced and the resulting decrease in space current through device |45 causes the device |52 to become conductive to positive signals impressed on control electrode |54. Hence, device |52 is conditioned to repeat positive pulses applied to control electrode |54, whenever the rectied control signal is impressed on control electrode |36 of device |3| simultaneously with the application of a pulse from the second blocking oscillator on control electrode |39 of this device.

Vertical synchronizing signal equalizer pulses are obtained from generator 9 and these pulses are frequency multiplied in multiplier 43 of Figure 1 to a frequency of 120 Ikilocycles per second, for example. The frequency multiplied vertical equalizer pulses, shown in curve F, are impressed continuously across terminals |51, one of these terminals being grounded and the other being connected to control electrode v|51: of device |52. The anode |55 of device |52 is connected to the positive terminal of source |23 through a load resistor |56 and a pair of output terminals 215 are connected to this anode and to ground. Device |52 repeats positive pulses applied to control electrode |54 only during such intervals when the control signal shown in curve 5G, and the pulse output of the second blocking oscillator shown in curve 5E, occur simultaneously, and during such intervals a burst of the multiplied equalizer pulses, shown in curve 5J, appears across output terminals 215.

Hence, it can be seen that the control signal from control tape 26 of Figure 1 is transmitted over conductors 32, and occurs once per revolution of the control tape and code tape, and lasts for a short interval. This control signal furthermore causes a burst of signal composed of multiplied equalizer pulses to be radiated on the video carrier once per revolution of these tapes. The phasing circuit 4| of Figures l and 4 operates as described, so that this burst of signal appears on a vertical blanking pulse, preferably on the portion of this pulse following the vertical synchronizing pulse.

It is necessary to distribute the key signal pattern recorded on the code tape 21 of the system of Figure 1, or the patterns on the code tapes 21a, 21h, 21o of the modication shown in Figure 1A, to the subscriber receivers. This distribution may be conveniently accomplished by suitable switching apparatus at the transmitter, or by a distributing station located near a group of subscriber receivers. Such a distributing station is shown in Figure 6, wherein line circuits 32a, 32h, 32e extending from the transmitter are brought into the central station as shown. The line circuits 32a, 32h, 32e respectively carry recurring key signal patterns corresponding to the patterns on the respective tapes 21a, 21h, 21c of Figure 1A; and these circuits are connected to selector switches |58a, |581), and others when so desired. The selector switches |5601l and |581) are connected respectively to switching devices |59a andlsb, and these devices are in turn respectively coupled to telephone head-sets |60a, |651). The coupling to the head-sets may be accomplished by inductive coupling between coils |6|a and i6 lb and the respective voice coils in the head-sets, or by other suitable means. The telephones are connected to subscriber receivers over the usual telephone networks and connection is made to these networks by lines |62a, |621).

The switching devices |59a and |5917 are connected to a pick-up head |63 associated with a tape |64, through respective switches |65a and |6529. A permanent signal is recorded on tape |64 by any known means, and this signal is impressed on devices |59a or |591) whenever switches |65a or |651) are closed. The devices |59a and |591) are similar to the switch device I8 of Figures 1 and 3, and these devices close the connection from selector switches |58a and |5813 to coils |6|a and |6|b whenever the signal from tape |64 is impressed thereon. 'I'he tape |64 is synchronized with the tapes 26, 21 and 28 of Figure 1, and is driven at a speed that is some sub-multiple of the speed of tapes 26 and 21, for example, at one revolution per minute. Synchronization between the tape |64 and the transmitter tapes is effected by means of a television receiver |66. This receiver is coupled to a suitable antenna and tuned to the television signal transmitted by the system of Figure 1. Vertical synchronizing pulses are separated from the received television signal and applied to a generator |61 which drives a synchronous motor |58, which in turn drives the tape 64 at the required speed. The length of the signal recorded on tape |64 is made such that when this tape is revolved at the required speed and switch |65a is closed, device 59a is caused to close for a time interval corresponding to one revolution of the code tape 21 of Figure 1. Similarly, when switch |651) is closed, device |591) is caused to close for a time interval corresponding to one revolution of the code tape 21.

Hence, some time before the program intervals of the transmitter of Figure l, a subscriber may receive key signal patterns to decode the television signal for one or more program intervals, whatever he desires. To do this the subscriber merely calls the central station, this call being received for example over the line |62a on the telephone lilla. An operator answers his call and instructs him to erase all signals from the tapes of his receiver (the receiver tapes and apparatus associated therewith are to be described herein at a later point). The operator then requests the subscriber to switch in the rst receiver tape, and the selector switch is then moved to select the line circuit 32a. Just prior to the coincidence of the leading edge of the signal on tape |64 with the pick-up head |63, the operator closes switch |65a, and leaves this switch closed until the trailing edge of the signal on tape |54 has passed the head |63, and then switch |65a is opened. In this manner, a single occurrence of the recurring key signal pattern on line 32a is transmitted to the subscriber receiver where it is recorded on a receiver tape. The operator then instructs the subscriber to switch in his second tape, and turns the selector switch |58a to select line 32h and the operation is repeated. By similar operations, the single occurrence of the key signal pattern on line 32C may be transmitted to the subscriber receiver, to be recorded on the third receiver tape.

Hence, the subscriber receiver may obtain one, two or three key signal patterns, each of which conditions his receiver to decode the television signals for a certain program interval` It is pointed out that such conditioning of the subscriberreceivers may be carried out several hours before the program intervals, so that at the beginning of the rst program interval, all the subscribers desiring to receive the ensuing programs are equipped with the necessary key signal patterns.

The whole operation of distributing the key signal patterns to a subscriber receiver takes place in a relatively short interval before the commencement of the program interval, and each subscriber uses the telephone network only rfor the short interval before the program interval and not continuously, as in previouslyvdisclosed systems. Many subscribers may be served over the telephone head-sets |60a and |601), and additional head-sets may be installed in the central station when the number of subscriber receivers in the locality of the station warrants such installation.

The apparatus of the central station has been described as being manually controlled by an operator. However, when so desired automatic control means may be used to effect the receiver switching operations in response to a request for the key signal patterns by a subscriber.

A subscriber receiver for receiving and decoding the composite television signal from the transmitter of Figure 1 is shown in Figure '7. 'Ihe receiver comprises a tuner and detector |69 which may be coupled to any suitable antenna |10. The tuner ,and detector is connected to a video amplifler |1| which in turn is connected to a usual image tube or reproducing device |12. Synchronizing signal components in the received television signal are separated therefrom by means of a synchronizing signal separator |13 coupled to the output terminals of detector |69 and fur- ,f

ther coupled to a vertical sweep generator |14, and to a horizontal sweep generator |18 through a delay line `and switch |15. The vertical and horizontal sweep generators are respectively connected to the vertical sweep coils |11 and horizontal sweep coils |18 associated with tube |12. The delay line and switch is controlled by a key signal filter and switch operator |19 coupled thereto by means of three leads |80. The switch operator |19 is also connected to the vertical sweep generator |14 by means of leads |8|, and to a pick-up head |82 associated with a receiver tape |83 by Way of leads |84.

In the operation of the receiver described to lthis point, the coded television signal from the system of Figure 1 is received on antenna |10, and tuned and detected in receiver stages |69. The video-frequency components of the detected signal are amplified in the amplifier |1| and are then impressed upon the reproducing device |12. in the usual manner, to control the intensity of the cathode ray beam in this tube. The synchronizing-signal components of the detected signal are separated therefrom in the separator stage |13, and ythe vertical synchronizing pulses are used `to synchronize the vertical sweep generator |14 with the received signal. The horizontal synchronizing pulses are applied to the horizontal sweep generator |15 through .a delay line and switch |15, and this delay line and switch operates in two positions, in a manner similar to the corresponding stage I4 of the transmitter of Figure 1, to pass the horizontal synchronizing .pulses undelayed in one position of the delay line and switch and to delay the passage of these '16 pulses in the other position thereof. The delay line and switch |15 is switched from one position to another by the key signal filter and switch operator |19, and the key signal filter and switch operator |19 is responsive to the coincidence of key signals from'tape |83 on leads |84 and vertical blanking pulses from the vertical sweep generator |14 on leads |8I. The Akey signal filter and switch operator is sharply tuned to be responsive to signals on leads |84 of the frequency of each key signal burst and to discriminate against other signal frequencies. As fully described in copending application Serial No. 773,848, when the key signals on leads |84 are properly timed with the key signals at the transmitter, delay line and switch |15 is operated in synchronism with the delay line and switch I4 of Figure 1. Hence, delay line and switch |15 acts to delay the horizontal scanning of the image tube |12 during the spaced intervals when the video-frequency components of the received television signal are delayed, and therefore acts to compensate for the delay and thus to decode the received television signal. Under these conditions, image tube |12 reproduces the intelligen-ce contained in the received television signal.

In contrast with the previously disclosed systems, the key signals are not impressed directly on the leads |84 over a line circuit extending to the transmitter. In the present system, to receive the key signal the subscriber calls the central station of Figure 6 by means of a telephone head-set |85 which is connected to the usual telephone network by a line |86. A recording coil |81 associated with tape |83 is shown coupled to the line |86 by way of a switch |88' and an inductance coil |88 coupled to the voice coil in the head set |85, although any known coupling or connecting means between the recording coil |81 and the line |86 may be used. An erasing signal source |89 is connected through a switch |90 to an erasing head 9| associated with the tape |83. After connection has been made with the central station, the subscriber is requested to erase any existing signal from the tape |83, and to do this he closes switch |90 for several revolutions of the tape. When the erasing is completed, and switch |90 is opened, 4a single key signal pattern is transmitted over the line circuit from the central station, as previously described, and is recorded on the tape |83. The recorded pattern is impressed on leads |84 by means of the pick-up head |82, and this pattern is used recurrently as a decoding signal to decode the received television signal until such time as the key signal pattern at the transmitter is changed.

'Ihe receiver tape |83 is driven `by a motor |89'. and during intervals when a new key signal pattern is being recorded on this tape the subscriber closes a switch 290 in the leads connecting the synchronizing signal separator |13 to a multivibrator |92. This causes the multivibrator |92 to be synchronized with the vertical synchronizing pulses of the received subscription television signal, and this multivibrator synchronizes a generator |96 at this frequency. 'I'he generator |96 in turn drives the motor |89' at this synchronous frequency, and therefore when the switch 260 is closed the receiver tape |83 revolves at a speed identical with that of the transmitting code tape 21 of Figure 1. Hence, the key signal pattern recorded on the receiver tape |83 17 is identical to the key signal pattern on the transmitter code tape 21 of Figure 1.

When the recording operation at the receiver is completed, the switch 268 is opened, the switch |86 is opened and the motor |89 is synchronized in the following manner: Vertical synchronizing signals from the separator |13 are impressed on the multivibrator circuit |92 by way of leads |93, a switch device |94, and leads |95. When the switch device |94 is closed the multivibrator |92 is synchronized at the frequency of the vertical synchronizing pulses. The multivibrator |92 in turn synchronizes a generator |96 at this frequency, and the generator drives the synchronous motor |89' at the required speed so that the receiver tape |83 revolves at the same rate as the code tape 21 of Figure 1.

Since identical key signal patterns are recorded on the transmitter and receiver tapes, and since these tapes are rotated at the same speed, as long as these patterns are in phase with each other, the television signal is coded at the transmitter and decoded at the receiver, and device |12 reproduces an undistorted television image. However, should receiver tape |83 fall out of step with the code tape 21 of Figure 1 due to the receiver being .shut off for a certain period, or for other reasons, the key signal pattern recorded on the receiver tape is no longer in phase with the key signal pattern recorded on the transmitter code tape and, hence, the receiver is no longer able to decode the coded signals. For this reason, means are provided for bringing the receiver tape back into step with the transmitter tape, and this is accomplished in a manner to be described. The control signal from control tape 26 of Figure 1 is received over line |86 with the key signal pattern, and this control signal is recorded on tape |83 together with the key signal pattern, and applied to the switch device |94 over leads |91. This control signal is also applied t switch operator |19 over leads |84, but as previously described the control sig-nal has a different frequenoy than the key signal and the switch operator |19 discriminates against and is unresponsive to this control signal. Similarly, the key signal is also applied to switch device |94 over leads |91, but the switch device |94 discriminates against all frequencies except that of the control signal. The equalizing pulses of multiplied frequency from control signal phasing circuit 4| of Figure l are selected from the received television signal by a conventional lter and rectifier |98, and the rectified output of filter and rectifier |98 is also impressed on switching device |94. A single time coincidence of a signal from filter and rectifier |98, and the control signal from leads |91, causes the switching device |94 to close and remain closed, thus passing the vertical synchronizing signals from leads |93 to leads |95 and hence to multivibrator |92. The multivibrator |92 is given a fre-running rate which is slightly lower than the frequency of the vertical synchronizing signals. When the receiver tape |83 is out of step with the transmitter code tape 21 of Figure 1, the subscriber depresses a push button, not shown, on switch device 94 which opens this device until the next coincidence of the control signal and signal from rectifier |98. When device |94 is open, multivibrator |92 oscillates at its free-running rate and synchronizes generator |96 at a slightly lower frequency than that of the vertical synchronizing signals, and therefore tape |83 turns at a slightly lower speed. When the control signal on tape 83 aligns itself with the control signal on tape 26 of Figure 1, the signal from filter' and rectifier |98 and the control signal from tape |83 occur simultaneously at switching device |94, and this simultaneous occurrence causes the switching device |94 to close and remain closed, thus passing the vertical synchronizing signals to the multivibrator |92, and the tape |83 immediately locks-in with the transmitter code tape. The receiver is, therefore, again conditioned to decode the received coded signals. As previously stated, switching device |94 is so arranged that when the control signal from tape |83 and the signal from the filter and rectifier |98 occur simultaneously, the switching device |94 closes and immediately passes vertical synchronizingpulses to the multivibrator |92, and continues to pass these synchronizing pulses. Hence, when the receiver tape |83 loses synchronism with the transmitter tape, the operator merely depresses the above mentioned pushbutton and in a short interval synchronism is restored.

The pick-up and recording head |82 may be made adjustable in a direction yalong the longitudinal axis of the tape |83, as indicated by the arrow |99. This allows for a slight adjustment to be made to compensate for time delays in the lines and circuits used in the transmission of the key signal pattern to the receiver.

In the receiver of Figure '7 as in the transmitter of Figure 1 a single code tape is used and the setting up of the pattern on the tape must be done some time before the program interval. When, as indicated in Figure 1A a plurality of code tapes is used at the transmitter, the key signal pattern may be changed between :program intervals by merely switching out one code tape and switching in another. To enable the receiver to decode the television signal when the key signal pattern is so changed at the transmitter, the receiver may be equipped with a plurality of tapes, so that different tapes may be switched into the receiver circuit to correspond with the changes at the transmitter. Such a system for the receiver circuit is shown in Figure 7A.

In the system of Figure 7A, the'synchronous motor |89 drives a plurality of receiver tapes |83a, |83b, |830. Any time before the program intervals the subscriber may call the -central station and receive the required patterns and, upon operation of a selector switch 200, may record these patterns respectively on the three tapes. The subscriber may close switch 28 la and use the pattern on tape |83a for the first program interval, and likewise may selectively close switches 28|b and 2|l|c for following program intervals. In each case the tape selected must have a key signal pattern recorded thereon which coincides with the key signal pattern used by the transmitter for that particular interval. Moreover, the change over from any one control tape to another may be achieved automatically by any conventional time controlled selector.

It is pointed out that with the modified systems of Figures 1A and '7A the transmitter may provide three or more patterns for three or more separate intervals, and the subscriber has the option of purchasing one or more of these patterns, depending on the number of prog-rams that he wishes to receive. k

switching device |94 is shown in detail in Figv ure 8, and referring to this figure, the input tervminals of the circuit are designated 262, and these terminals are connected to the leads |91 and "acta-iov hence 11.0, Pics-ue Oeil '1.8.2. of Fleurs 1- 0.11 ef tneterniineis zu: is grounded and the other is coupled `to a control electrode 203 of an electron discharge device 20,4 through a. coupling capacivto;` Ztl-'lycontrol electrode 203 being connected to ground through a grid leak resistor 20E. The cathodev A211'1 of device 204 is. connected directly to ground, and the anode zus of this device is 'connected to the positive terminal of a source 'discharge device 215, the common junction between rthe cathode of device 213- and control electrode 2 14 of device 215` being connected to ground through a resistor 216, and this resistor being s hunted vby a capacitor 2-11. The device 213 conducts until, capacitor 21'1 is charged, and thereafter at a certain point of each revolution of the receiver tape 113,3` 0i] Figure '7, the control v signal recordedy thereon appears at anode 208 or device 2114-, and in response to this signall a positive potential appears across resistor 216. Cathode 218 ofy device 215 is connected to ground through a cathode resistorv 219, this resistor being shunt'ed by a capacitor 220. Cathode 21B is'further connected to the positive terminal of source 209l through a` resistor 221. The screen electrode'222 of device 2 15 isj connected to the positive terminal of source 209l through a resistor 223, and thisv electrode is coupled to ground through a capacitor 224, The rectiedequalizing pulses of multi-plied frequency from lter and rectifier 1'98 of Figure 7- are impressed across terminals 225'; one of these terminals being connected tog-round, andthe other being coupled to a second control electrode 226 of device 215 through a capacitor 221. Control electrode 225 isconnected to ground through a grid leak resistor 228. The anode 229 of device 215 is connectedlto the positive terminal of source 209 through a loady resistor 230. The device 215 is normally biased to cut-off by the positive p otential on cathode 2'11!y dueto the potential di- Avider action ofV resistors 221V and 219, and the circuit of this device is so arranged that .the device` becomes conductive only When the control electrodes 214 and 226 are simultaneously driven positive. As previously described, this condition 'occurs only when the control signal on control tape 2-5 of\Figure 1 andthe control signalon the receiver tape 1-83 of Figure 7 ocour simultaneously, that is, when the tapes at the transmitter-andrai'. the receiver are in step.

The anode 2290i; device 215 is connected to ar-"control electrode 231 of an electron discharge device-:2132, this control electrodebeing connected through'- aresistor-fV 233 to the negative terminal ofv av biasing source- 234, the positive terminal offthis sourcebeing grounded. Cathode 235 of device 232- i's connec1.'edV to ground, and the anode 236 of this device is connected to the positive terminal of source 209 through a resistor 231. The\anode236of device 232 is further connected to theec'ontrol" electrode 238 ofv an electron discharget device 239 throughv aresistorA 240, this .on control electrode 231. of device 232. 2132; therefore, becomes highly conductive kand 20 controlelectrode vbeing connected to the negative terminal of source 234 through a resistor 241, and this resistor being shunted by a pushbutton 242. Cathode 243 of device 239 is connested to ground, and the anode 244 thereof is connected to the positive terminal of source 209 through a resistor 245. The anode 244 of d'evice 239 is further connected to the control electrode 231 of device 232 through a resistor 245. The control electrode 238 of device 239 is further connected to the control electrode 241 of another electron discharge device 248. The cathode 249 of dev-ice 248 is connected to ground through a resistor 2150, and the anode 251 of this device is connected to the positive terminal of source 209 through a resistor 252. Vertical synchronizing signals from synchronizing signal separator 113 of Figure 7 are impressed across terminals 2513, one of these terminals being grounded and the other being connected to cathode 249 of device 248. When device 248 is conductive, the vertical synchronizing signals irnpressed across terminals 253 appear across output terminals 254, one of the terminals 254 being groundedv and the other being connected to anode 25.1 of device 248. Terminals 254 areconvnected to the multivibrator 192 of Figure 7 by way of leads 195.

In normal operation, the value of the negati-ve potentialof source 234 is made sufficient to overcome the positive bias on control electrode 231 and drive this control electrode negative, and. hence device 232 is non-conductive and the relatively high potential value of anode. 236 impresses a positive potential on control electrode. 238I of device 239, by reason of the connection of anode 236. to control electrode 23B through resistor 240. This positive potential overcomes' the negative bias on control electrode 238 from source 234, and causes device 239 to be conductive. Control electrode 2.38 of device 239 is connected to control electrode 247 of device 248, and hence. in this condition control electrode 241i has a potential impressed thereon sufficiently positive tol make device 248 conductive. Hence, vertical synchronizing signals impressed across terminals 253 are conducted by device 248 and appear across terminals 254. Therefore, vertical' synchronizing signals from the synchronizing signal separator 1'13 of Figure 'lf are passed bythe switching device 194 to the multivibrator 192,. and the receiver tape 183 is rotated in synchronism with tape 21 of Figure l. Should synchronism between. the transmitter and receiver tapes. be lost, it is merely necessary. for the operator to press. push-button 242 for a. moment `and then release this button. When push-button 242' is depressed, negative biasing potential from sourcev 234i is applied directly to control electrode 238 of device' 239. Thispotential is sufficient to drive device 239 to cut-off,- and'v the `resulting rise in anode potentialof device 239 overcomes the negative bias Device `trode124.'1t'.o be such that device 248 is nolonger conductive. chronizingsignal's' are impressed on multivibrator In this condition no vertical syn- 12921" of :Figure- 7; ande the. multivibrator' in itsY free- 

